MALABO, Dec 6 (Reuters) - Heavily armed gunmen stormed two banks simultaneously in Equatorial Guinea's second city Bata, grabbing bags of cash and shooting passers-by before making off in speedboats into the Atlantic Ocean, the government said.

Armed with light automatic weapons, the bandits arrived in the city by boat disguised as fishermen before carrying out the well-planned raid, the government said in a statement read on state television late on Wednesday.

After exchanging fire with the masked attackers, Equatorial Guinean security forces pursued them until they crossed into the territorial waters of neighbouring Cameroon, it said.

Sources in the region said a U.S. national and his driver escaped unhurt when their vehicle was hit by bullets.

Parts of the Gulf of Guinea, home to some of Africa's biggest oil reserves, are notoriously dangerous, with heavily armed bandits and militants in Nigeria's oil-rich Delta using boats to attack offshore oil platforms.

Until now Equatorial Guinea, one of the world's fastest-growing economies in recent years thanks to oil fields developed since the mid-1990s, has been relatively unaffected.

Gunmen suspected of belonging to a faction of Nigerian Delta militant group MEND killed 21 Cameroonian soldiers on Nov. 12 in the long-disputed Bakassi peninsular between the two countries.

That attack raised fears of a widening of MEND's campaign, which has previously targeted Nigerian forces and local and foreign oil workers, although the group denied involvement.

Equatorial Guinea, a former Spanish colony of barely half a million people, is split between the Atlantic island of Bioko, where the capital Malabo is situated, and a territory on the African mainland where Bata is located. (Reporting by Bernardino Ndze Biyoa and Estelle Shirbon; writing by Alistair Thomson)

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